OK, just checked in at Southwest in West Palm Beach (PBIA), and it was perfect!
The agent got the declaration form, had me fill it out and open my bag, then got a roll of clear tape and taped the form to the outside of my Secure-it/Center-of-Mass lock box. I told her that when I checked in at SWA in BWI, the agent had me put the form inside
the lock box. I asked the BWI agent how can the TSA see that I've properly declared the firearm?, and she had the brilliant response "I guess they can see inside".
The agent at PBIA complimented me on my lockbox and said she liked how it was secured to the frame of my luggage. She said I wouldn't believe some of the boxes people try to use that completely compromise the security of the firearm.
One new wrinkle - she didn't have me escorted downstairs to the entrance to the TSA area by baggage claim. She just put the bag on the conveyor behind the counter.